An organic EL device is a spontaneous light emitting device which utilizes the principle that a fluorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied. Since an organic EL device of the laminate type driven under a low electric voltage was reported by C. W. Tang of Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, Pages 913, 1987), many studies have been conducted on organic EL devices using organic materials as the constituting materials. Tang et al. used a laminate structure using tris(8-hydroxyquinolinol)aluminum for the light emitting layer and a triphenyldiamine derivative for the hole transporting layer. Advantages of the laminate structure are that the efficiency of hole injection into the light emitting layer can be increased, that the efficiency of forming excited particles which are formed by blocking and recombining electrons injected from the cathode can be increased, and that excited particles formed within the light emitting layer can be enclosed. As the structure of the organic EL device, a two-layered structure having a hole transporting (injecting) layer and an electron transporting and light emitting layer and a three-layered structure having a hole transporting (injecting) layer, a light emitting layer and an electron transporting (injecting) layer are well known. To increase the efficiency of recombination of injected holes and electrons in the devices of the laminate type, the structure of the device and the process for forming the device have been studied.
In general, when an organic EL device is driven or stored in an environment of a high temperature and a high humidity, adverse effects such as a change in the emitted color, a decrease in the efficiency of light emission, an increase in the voltage for driving and a decrease in the life of light emission arise. To prevent the adverse effects, it has been necessary that the glass transition temperature (Tg) of the hole transporting material be elevated. For this purpose, it is necessary that the hole transporting material have many aromatic groups in the molecule (for example, aromatic diamine derivatives described in Patent Reference 1 and aromatic condensed ring diamine derivatives described in Patent Reference 2) and, in general, compounds having a structure having 8 to 12 benzene rings have been preferably used.
However, when many aromatic groups are present in the molecule, crystallization tends to take place during the formation of a thin layer using the hole transporting material in the preparation of an organic EL device, and the crystallization causes problems in that the outlet of a crucible used for the vapor deposition is clogged and that defects caused by the crystallization are formed in the thin layer, and the yield in the production of the organic EL device decreases. The compound having many aromatic groups in the molecule causes a short life, in general, since the compound has a high temperature of sublimation and problems such as decomposition of the compound during the vapor deposition and uneven vapor deposition arise although the compound has a high glass transition temperature (Tg).
Asymmetric aromatic amines are disclosed in references. For example, an aromatic amine derivative having an asymmetric structure is described in Patent Reference 3. However, no specific examples are described, and characteristics of the asymmetric compound are not described at all. An asymmetric aromatic amine derivative having phenanthrene is described in Patent Reference 4. However, the asymmetric compound is treated in the same manner as symmetric compounds, and characteristics of the asymmetric compound are not described at all. The synthesis of an asymmetric compound requires a special synthetic process. No processes for synthesis of the asymmetric compounds are clearly described in the above Patent References. A process for producing an aromatic amine derivative having an asymmetric structure is described in Patent Reference 5. However, characteristics of the asymmetric compound are not described at all. A thermally stable asymmetric compound having a high glass transition temperature is described in Patent Reference 6. However, the compounds described as the example are limited to compounds having carbazole. When a device using this compound was prepared by the present inventors, a problem was found in that the device had a short life.
As described above, although organic EL devices having a long life have been reported, no devices had a sufficient life. Therefore, an organic EL device having more excellent properties has been strongly desired.    [Patent Reference 1] Specification of U.S. Pat. No. 4,720,432    [Patent Reference 2] Specification of U.S. Pat. No. 5,061,569    [Patent Reference 3] Japanese Patent Application Laid-Open No. Heisei 8(1996)-48656    [Patent Reference 4] Japanese Patent Application Laid-Open No. Heisei 11(1999)-135261    [Patent Reference 5] Japanese Patent Application Laid-Open No. 2003-171366    [Patent Reference 6] Specification of U.S. Pat. No. 6,242,115